1. Field of the Invention
This invention relates to energy storage and conversion devices. In particular, this invention relates to energy storage and conversion devices using thin film electrodes manufactured by thermal spray.
2. Description of the Related Art
Energy storage devices, such as batteries and super capacitors, and energy conversion devices, such as fuel cells and thermoelectrics, both require electrodes comprising an active material for the energy storage, conversion, and/or release processes. Each year, billions of dollars are spent on both primary and rechargeable batteries for use in applications ranging from small batteries for portable electronics and communications equipment, to larger batteries used for automobiles and uninterruptible power supplies (UPS).
The LiSi/FeS2 couple is the primary power source used for thermally activated batteries (xe2x80x9cthermal batteriesxe2x80x9d) for some nuclear weapons and missiles, as described in U.S. Pat. Nos. 4,119,769, 4,840,859, and 4,675,257, which are incorporated by reference herein. These batteries are designed to function only when the electrolyte phase becomes molten. Until the internal pyrotechnic heat source is ignited, the batteries are inert and have an almost unlimited shelf life. Common electrolytes used for these applications include the LiClxe2x80x94KCl eutectic that melts at 352xc2x0 C. and the all-lithium LiClxe2x80x94LiBrxe2x80x94LiF minimum-melting electrolyte that melts at 436xc2x0 C.
The cathode, separator, and anodes for thermally activated batteries are typically prepared by cold pressing of powders in dies to form pellets. The separator contains enough MgO (typically, 35 weight %) to act as an immobilization agent for the electrolyte once the battery has been activated and the electrolyte melts. The catholyte contains 25% or more of separator material and, in many cases, 1.5% Li2O to act as a lithiation agent to mitigate voltage transients caused by electroactive iron impurities. The anode contains 20-25% electrolyte to aid in cold pressing and to improve the electrochemical performance by increasing the ionic conductivity. Each cell in a bipolar thermal battery stack contains pellets of anode, separator, cathode, and pyrotechnic source (typically, Fe/KClO4 blends). Stainless steel current collectors are located between the anode and the heat pellet, and between the heat pellet and cathode of the adjacent cell. The current collectors also serve as thermal buffers to moderate the heat input to the active cell components. This is important for the FeS2 (pyrite) cathode, in that FeS2 becomes thermally unstable above 550xc2x0 C., decomposing according to equation 1:
2FeS2xe2x86x922FeS+S2(g))xe2x80x83xe2x80x83[1]
Under these conditions, the fugitive sulfur vapor can react with the LiSi anode to generate enough heat to cause a thermal-runaway condition, where the battery self-destructs.
The need to press catholyte powders such as FeS2 into thin films or pellets for use in thermal batteries increases production costs because of the high labor costs associated with processing of the material (e.g., blending, pelletizing, and quality control checks for weight and thickness). While the current technology of using cold-pressed pellets is suitable for its intended purposes, it has a number of intrinsic limitations. The thinnest pellets that can be fabricated with reasonable yields are in the range from 0.010 to 0.012 inches in thickness. For many applications this results in using a far greater amount of material than is actually needed. A graphite-paper substrate may be used as a reinforcing agent in the cathode pellet, but is not an option with the separator and anode pellets. The use of excess material increases the length and mass of the thermal battery unnecessarily. Pressing of large pellets becomes increasingly more difficult as the diameter of the pellet is increased from 1 inch to 5 inches. The necessary pressure for compaction of pellets increases rapidly as the square of the area of the pellet, so that presses with capacities of 500 tons or more are needed for the larger pellets. Such large presses are very expensive.
Thin films, and thin film electrodes in particular have been fabricated by other techniques, including spray pyrolysis and chemical vapor deposition (CVD). Thin film electrodes have also been prepared by sol-gel methods, electrochemical methods, and molecular beam deposition. An approach to fabrication of electrodes by thermal spray has been reported by R. Henne (Institute far Technische Thermodynamik, Stuttgart, Germany) for at least one energy conversion device, a solid oxide fuel cell, wherein yttria-stabilized zirconia (YSZ) and porosity-graded perovskite are deposited by direct current plasma spray to fabricate multilayer structures. R. Zatorski of Sulzer-Metco has also reported production of battery electrodes by thermal spray techniques. However, the above reports are directed to thermally stable materials which do not decompose at the high temperatures generally required for thermal spray.
Synthesis of thin films of pyrite in particular has previously been investigated. G. Pimenta et al. have produced pyrite using H2S-reactive iron. Pyrite and pyrite films have also been prepared by chemical vapor deposition, sulfurization of iron oxides, electrodeposition of iron films, argon and reactive sputtering, screen printing processes, and physical vapor deposition. Conventional and fine pyrite (micron-sized) is also produced in aqueous solution.
Commonly assigned, copending U.S. patent application Ser. No. 09/432,334 titled Energy Storage and Conversion Devices Using Thermal Sprayed Electrodes and filed on Nov. 2, 1999, discloses the use of sulfur as a thermally protective barrier coating for the active material when making an electrode by thermal spray. The sulfur barrier coating protects the encased active material from the heat of thermal spray thus preventing decomposition. The use of sulfur has some drawbacks however, namely the presence of free sulfur in the electrode. Free sulfur contributes to the initially high voltage seen when the cell starts to discharge, and in some cases free sulfur must be removed, typically by leaching with carbon disulfide, a highly flammable material, thus complicating electrode processing.
A method for the manufacture of an electrode for an energy storage or conversion device comprises thermally spraying a feedstock mixture comprising an effective quantity of a source of a thermally protective salt and an active material or active material precursor onto a substrate to produce a film of the active material and salt. The film can have a thickness of about 1 to about 1000 microns.
In a particularly advantageous feature, useful active materials include materials which ordinarily decompose or are unavailable at the high temperatures used during thermal spray processes, such as metal chalcogenides. The active materials may be thermally sprayed to form an electrode when the feedstock mixture employs an effective amount of a source of the thermally protective salt coating. The active material feedstock may comprise microstructured or nanostructured materials, which after thermal spray results in electrodes having microstructured or nanostructured active materials, respectively.